Kraft pulp is produced by cooking wood chips in an aqueous medium mainly comprising sodium hydroxide and sodium sulfide in high temperature under high pressure to remove lignin from the chips. The typical composition of the cooking liquor (white liquor) is 55 to 100 g/l of sodium hydroxide, 18 to 45 g/l of sodium sulfide, and 10 to 30 g/l of sodium carbonate, each in terms of Na.sub.2 O, which is sometimes used as partly diluted with a black liquor. The cooking temperature is about 170.degree. C.
The calcium ion dissolved out of wood chips reacts with a carbonate ion in the cooking liquor to form calcium carbonate in the system, which is precipitated and deposited on the inner wall of a digester and subsequent tanks and pipes. Calcium carbonate scale deposited in various zones of a continuous digester, especially in the upper cooking zone, the heat exchangers and the digester screens causes various operational problems, such as reduction in thermal efficiency, hindrance to the flow of the liquid and pulp, reduction in productivity, and non-uniform pulp quality, and necessitates frequent cleaning.
The scale is generally removed by planned cleaning with an acid solution. This method is, however, disadvantageous in that scale removal itself takes much labor and that suspension of operation causes a production loss and an enormous energy loss.
It has therefore been demanded to develop a technique for continuously controlling scale deposition and thereby minimizing the necessity of scale removal. In this line, it has been proposed to use a maleic acid polymer (see JP-B-2-53551, the term "JP-B" as used herein means an "examined published Japanese patent application") or polyaminopoly(alkylenephosphonic acids) and nonionic surfactants of polyalkoxyalkylphenols (see U.S. Pat. No. 4,799,995) as a scale deposit inhibitor. However, none of the inhibitors proposed gives satisfactory results for control of scale deposition, and there still has been a demand for a scale deposit inhibitor of higher effect.